Northwestern and Michigan State exposed the underbelly of Greg Mattison’s tough-minded defense.

Just run around the four upperclassmen on the line, away from the physical Kenny Demens and the athletic Jake Ryan and Brandin Hawthorne. Just run away from Jordan Kovacs.

Take the ball to the perimeter and watch the Michigan defense fall apart.

“I would attack our perimeter,” said Michigan coach Brady Hoke. “That’s where I would start.”

Northwestern used the speed option to get outside with two fleet-footed quarterbacks in Dan Persa and Kain Colter. The Spartans used a stretch running play to push Edwin Baker to the perimeter, where he found plenty of running room, racking up 167 rushing yards. Both used quick-hitting screen passes, matching up defenders one-on-one.

“I don't think we tackled worth a darn,” Hoke said Oct. 17, the Monday after the Michigan State loss.

“I don't think we played the perimeter of our defense worth a darn.”

Mattison told CBSSports.com that he wants his defense to have “eight or less” missed tackles per game. According to the report, Michigan has averaged six missed tackles per game and has reached Mattison’s goal in all but two games this season — Notre Dame (11) and Michigan State (18).

You could blame the defensive line for being on the ground, watching Baker run by after it couldn’t chop block the offensive line. Or you could blame the poor tackling of the cornerbacks — senior defensive tackle Mike Martin said the Spartans were “aware that they (were) capable of missing tackles.” What about the linebackers’ hesitancy? Or the unnaggressive play of the entire defense? But particularly, as Hoke said, the linebackers need to read the play quicker and react faster.

Mattison and Hoke used the bye week to fix these problems, because Hoke expects Michigan’s perimeter to be attacked again this week against Purdue.

The same stretch play that Michigan State ran with Baker and similar bubble and quick-hitting screen passes that both teams ran could come back to haunt the Wolverines, Hoke said, because Purdue runs the same plays.

“I think their backs are fast,” Mattison said, referring to junior running backs Ralph Bolden and Akeem Shavers. “They seem very, very quick. They have a lot of ways that they try to get to the perimeter with the ball.”

Added Martin: “They do a good job of getting the ball in a lot of guys hands. … They have a lot of weapons. We’ve just got to make sure we execute on defense and we run to the ball and get guys down.”

The problems that have plagued the whole defense — a lack of aggressiveness getting off blocks, everyone running to the ball, missed tackles — are worrisome because they are all fundamental basics of Mattison’s defense and trademarks of the unit during the Wolverines’ first five games.

In practice this week, the defensive players worked on wrapping up, running through offensive players and finishing tackles. If they did it wrong, the coaches ripped them and forced them to stay after practice to hit the sleds.

Running to the ball is a combination of aggressiveness to get off the initial block and awareness to contain, meaning that Michigan instructs every player to funnel the ball back to the middle of the field to another defender.

“We’ve seen what’s happened on film,” said fifth-year senior defensive tackle Will Heininger. “We just have to turn the ball in. Every coordinator I’ve ever had in high school, it was, ‘Turn the ball inside to your guys, and then have everyone pursue.'

“And we’re not good enough — we don’t have all 11 NFL players where everyone can make the play — we’ve got to get everybody to the ball and everyone do their assignment. (Then) it doesn’t matter if they run inside or outside.”

Purdue junior quarterback Caleb TerBush has been the facilitator of the equal-opportunity Boilermaker offense, which features two 300-yard rushers and three 200-yard receivers. During the bye week, many Michigan players watched TerBush play one of his best games of the season, as he completed 64 percent of his passes and spread the ball to eight different receivers. Purdue upset No. 23 Illinois, thanks in large part to TerBush.

But Michigan practiced all week with the mindset that fixing its own defense’s problems was its main concern.

“This game is about Michigan,” Martin said. “Coach has really been talking about that. Because it doesn’t really matter what Purdue does. … It boils down to what we do and how we execute.”

Hoke said his team practiced faster and more downhill this week, as the defense returned to its fundamentals.

“If you’re hesitant at all, that half a step that you’re behind can cost you as a defense," Hoke said. “I think we played a little bit tentative, and you can’t play tentative. If you’re going to make mistakes, make them aggressive. I can handle that.”